The invention relates to a process for the removal of organic compounds from water.
In the removal of organic compounds from water, the instance to improve the odor and taste of drinking water or to eliminate harmful substances in waste water from industries, active carbon is often used as the adsorbent. As a rule, the purification with active carbon is effected as the last step after one or more preceding purification steps, such as settling, filtration and similar processes. For the removal of organic compounds from water by adsorption on active carbon, the active carbon may be used in the form of a fixed bed as well as in fluidized form. After the active carbon has been loaded with organic compounds, it is usually regenerated, by which treatment the organic compounds are removed from the active carbon. The regeneration is usually performed by a high temperature treatment with an oxygen-containing gas such as flue gas or a mixture of steam and air. For obtaining an effective removal of the organic compounds temperatures higher than 400.degree. C. should be used in the regeneration.
The use of active carbon for the removal of organic compounds from water has three drawbacks. The principal drawback is connected with the nature of the organic compounds to be removed; the other two drawbacks are connected with the regeneration and the adsorption process. As to the nature of the organic compounds to be removed the following should be remarked. Active carbon shows a high adsorption capacity when it is used for removing organic compounds from water, which compounds have a low solubility in water, such as hydrocarbons. However, for the removal of organic compounds from water, which compounds contain in addition to carbon and hydrogen at least one oxygen and/or nitrogen atom and which organic compounds have in addition a solubility in water at 20.degree. C. of more than 1g/100 ml, the adsorption capacity of active carbon is very unsatisfactory. For the sake of brevity, organic compounds which contain in addition to carbon and hydrogen at least one oxygen and/or a nitrogen atom and which organic compounds have, in addition, a solubility in water at 20.degree. C. of more than 1g/100 ml, will in this patent application further be designated as organic hetero compounds. As regards the regeneration the following should be remarked. In the regeneration of active carbon by a high-temperature treatment a large amount of heat is liberated. In view of ineffective temperature control, the regeneration of active carbon in a fixed bed is not suitable. Therefore, the regeneration is usually carried out on active carbon which is in fluidized form. In spite of this precaution, in each regeneration treatment a considerable amount of the active carbon will be lost by combustion. As active carbon is a soft material, its particle size will decrease appreciably due to attrition when the carbon is regenerated in fluidized form, so that after each regeneration treatment a considerable amount of the active carbon has too small a particle size to be suitable for further use. As regards the adsorption process the following may be said. For the sake of simple processing, adsorption processes with alternately an adsorption step and a regeneration step are preferably carried out in such a way that the adsorbent has the same form in both steps, i.e. in both steps either the form of a fixed bed or the fluidized form. As stated hereinbefore, both embodiments are suitable for the adsorption step in the separation of organic compounds from water by means of active carbon. When the adsorption step is carried out using active carbon in the fluidized form, the particle size will decrease appreciably due to attrition, just as with the regeneration of active carbon in the fluidized form, so that after each adsorption step a considerable part of the active carbon will have too small a particle size to be suitable for further use. In this connection distinct preference is given to carrying out the adsorption over active carbon which is present in the form of a fixed bed. However, simple processing as described above, in which the adsorbent has the same form in both treating steps, is not possible then. In view of the problems connected with the regeneration of active carbon, in daily practice regeneration is often omitted. A process in which active carbon is used only once is of course very costly.
It will be clear from the above that for the removal of organic hetero compounds from water an adsorbent is needed that does not have the drawbacks connected with the use of active carbon. Applicants have carried out an investigation concerning this subject. As the aim of the investigation was to find an adsorbent having, inter alia, a thermal stability sufficient to enable the adsorbent to be regenerated by high-temperature treatment without appreciable combustion losses, the investigation was restricted to inorganic materials. Both amorphous and crystalline inorganic materials were included in the investigation. Amorphous alumina, amorphous silica and the crystalline aluminosilicates mordenite and faujasite were found to be unsuitable for the present purpose. These materials do have a thermal stability that is sufficiently high to make them eligible for use, but their adsorption capacity for organic hetero compounds is still considerably lower than that of active carbon.
It has now been found that certain crystalline iron silicates which have recently been synthesized for the first time, as described in U.S. Pat. No. 4,208,305 incorporated by reference, are pre-eminently suitable for use as adsorbent for the removal of organic hetero compounds from water.